Acoustic ink printers of the type to which this invention is addressed typically comprise one or more rf acoustic radiators for illuminating the free surface of a pool of liquid ink with respective acoustic beams. Each of these beams usually is brought to focus essentially on the free ink surface at a near normal angle of incidence. Furthermore, printing conventionally is performed by independently modulating the rf excitation of the acoustic radiators in accordance with the input data samples for the image that is to be printed. This modulation enables the radiation pressure which each of the beams exerts against the free ink surface to make brief, controlled excursions to a sufficiently high pressure level for overcoming the restraining force of surface tension. That, in turn, causes individual droplets of ink to be ejected from the free ink surface on demand at an adequate velocity to cause them to deposit in an image configuration on a nearby recording medium. Acoustic ink printing is attractive because it does not rely upon nozzles or small ejection orifices, which means that it alleviates some of the mechanical constraints that have caused many of the reliability and picture element ("pixel") placement accuracy problems conventional drop on demand and continuous stream ink jet printers have experienced.
Several different acoustic radiators (sometimes also referred to as "droplet ejectors") have been developed for acoustic ink printing. More particularly, there already are acoustically illuminated spherical acoustic focusing lenses (as described in a commonly assigned United States patent of Elrod et al., which issued June 14, 1989 as U.S. Pat. No. 4,751,529 on "Microlenses for Acoustic Printing"); piezoelectric shell transducers (as described in a United States patent of Lovelady et al., which issued Dec. 24, 1981 as U.S. Pat. No. 4,308,547 on "Liquid Drop Emitter"); and planar piezoelectric transducers with interdigitated electrodes (as described in a commonly assigned United States patent of Quate et al., which issued Sept. 29, 1987 as U.S. Pat. No. 4,697,105 on "Nozzleless Liquid Droplet Ejectors"). This existing droplet ejector technology is believed to be adequate for designing various printhead configurations, ranging from relatively simple, single ejector embodiments for raster output scanners (ROS's) to more complex embodiments, such as one or two dimensional, full page width arrays of droplet ejectors for line printing.
There still, however, is a need for sharply focused acoustic radiators which are easier and less expensive to manufacture in compliance with relatively exacting design specifications for applications, such as acoustic ink printing, requiring substantial predictability. There also is a need for less costly arrays of precisely positioned acoustic radiators. Moreover, the performance and reliability of some acoustic ink printers would be enhanced if the output faces of their acoustic radiators had more uniform ink flow characteristics, while other acoustic ink printers would benefit if the output faces of their acoustic radiators were easier to planarize.